Nanostructured alkaline-cation-containing δ-MnO₂ for photocatalytic water oxidation

Oxygen evolution from water is one of the key reactions for solar fuel production. Here, two nanostructured K-containing δ-MnO₂ are synthesized: K-δ-MnO₂ nanosheets and K-δ-MnO₂ nanoparticles, both of which exhibit high catalytic activity in visible-light-driven water oxidation. The role of alkaline cations in oxygen evolution is first explored by replacing the K⁺ ions in the δ-MnO₂ structure with H⁺ ions through proton ion exchange. H-δ-MnO₂ catalysts with a similar morphology and crystal structure exhibit activities per surface site approximately one order of magnitude lower than that of K-δ-MnO₂, although both nanostructured H-δ-MnO₂ catalysts have much larger Brunauer-Emmett-Teller (BET) surface areas. Such a low turnover frequency (TOF) per surface Mn atom might be due to the fact that the Ru²⁺(bpy) ₃ sensitizer is too large to access the additional surface area created during proton exchange. Also, a prepared Na-containing δ-MnO ₂ material with an identical crystal structure exhibits a TOF similar to that of the K-containing δ-MnO₂, suggesting that the alkaline cations are not directly involved in catalytic water oxidation, but instead stabilize the layered structure of the δ-MnO₂.